MEMORANDUM



MEMORANDUM

To: Distribution

From: F. Dylla/grn

Subject: FEL Upgrade Project Weekly Brief - February 5-9, 2007

Date: February 9, 2007

Highlights:

The Laboratory held a friendly roast on Friday afternoon in honor of Fred Dylla in anticipation of his departure. Fred will be greatly missed but you couldn’t always tell that from the stories people told.

In the FEL facility we spent two days studying the limitations to stable beam transport and believe we have identified the primary bad actors in the mirror train. We lased on our new set of 1.6 micron mirrors and found excellent performance. We then performed an extensive series of measurements for the Short Rayleigh Range JTO programs and discovered several features supporting the existing models and a few new interesting tidbits to consider. See the Operations discussion below for details.

We also continued to make significant progress in pulling together hardware for the gun test stand.

Management:

We hosted this week our Military Technical Advisory Committee for advice on solidifying the path forward to a 100 kW FEL.

Budget estimates and schedule planning were developed for the FY’08-09 Prioritized Task Statement provided by ONR. We will provide that information to ONR and visit them late next week for further discussions to develop supporting documentation for the amended MOU to be signed between ONR and DOE covering ongoing efforts in the next two fiscal years.

Planning for the remainder of FY’07 is still tentative as we await the outcome of the continuing resolution and its impact on CEBAF cryogenic operations. We will provide an update of plans when that settles down.

Operations:

After checking out the 1.6 micron lasing performance at "low" power on Friday we had a brief down to hi-pot on ion pump that was closing the high reflector mirror can valve.  This was extremely successful and we have not had any vacuum problems in that region since.  We also tested out a viewer on the 0G dump line to measure the OTR output at 9 MeV/c.

On Monday and Tuesday we ran some tests of the optical transport to User Labs 1 and 2 to study issue with delivering stable high-power beam to those labs.  The new 1.6 micron output coupler performed well on Monday.  The FEL could deliver over 2.5 kW to the end of line with 2.5 mA of beam current.  This is over 3.1 kW from the laser itself.   The power was also delivered to both User Labs 1 and 2.  We found that there were large losses on the two mirrors sending the beam into Lab 1 but that the mirrors sending the beam into User Lab 2 were fine.  A summary of the beam measurements is given in the Optics section.  The laser did not perform as well on Tuesday.  We had a problem with some of the accelerator phases drifting around and that might have reduced the efficiency somewhat.  On both Monday and Tuesday we delivered up to 1.4 kW of laser power for a couple of short runs to the NASA/W&M  nanotube experiment for production of boron-nitride nanotubes. These runs went well.  Samples are being analyzed.

Wednesday and Thursday were devoted to taking data on JTO contract studying the physics of FEL oscillator operation with a short Rayleigh range.  We set up for the experiment on Wednesday and took careful data on the Rayleigh range vs. deformable mirror heater power.  This is a very difficult and critical measurement for the short Rayleigh range studies since it provides the baseline for all other data collection.  On Thursday we took data on the gain, loss, and power, as a function of mirror heater power for both full charge beam and half charge beam.  We also tried optimizing the focus while the laser had a short Rayleigh range and found a 5 to 10% improvement in gain.  This is close to what is predicted from Colson's  simulations.  Finally we had a chance to qualitatively study the sensitivity to mirror steering.  Previously we found that the gain was not strongly reduced if the mirror tilt did not move the laser mode by more than one full divergence of the beam.  This is already quite robust and not what one expects from back-of-the-envelope calculations.  More surprising still was that the efficiency did not seem to vary much at all as long as the laser mode stayed on the  mirror.  This means that the main problem with mirror angular jitter is pointing stability and not power jitter.

Today we have a down day while some optics maintenance and repair work is done.  We will be re-cesiating the cathode on Monday before we come up.

WBS 4 (Injector):

The photocathode delivered a total 473 Coulombs and 81 hours of CW beam time since the last re-cesiation on January 12, 2007. The photocathode will be re-cesiated next Monday. The long lifetimes we have been obtaining recently are believed to be associated with over-cesiating the photocathode at the expense of obtaining high QE.   A new wafer was mounted in the test chamber for continuing our research on scatter light as a function of temperature during the heat clean cycles. The molybdenum anode plate for the test stand gun has been flattened by a mechanical grinder in preparation for polishing. This plate will be used also as a mirror for directing the drive laser beam onto the photocathode. We started to assemble electrical components for the remote stalk retractor system. This system will shorten the re-cesiation time from 3.5 to 0.5 hours in the FEL gun. The installation working in the Gun Test Stand vault positioned the high voltage tank and continued to make progress installing the hydraulic arm for the concrete shield door. Survey and alignment crew is working on some issues with their previously set reference coordinates before aligning the tank. We are starting to design the SF6 transfer system layout for the Gun Test Stand.

WBS 8 (Instrumentation):

There was progress made on many fronts this week! We are keeping the momentum for operator training and documenting procedures from the previous two weeks. A comprehensive document is being prepared that combines all of our procedures and will be posted on line for reference. We have installed continuously variable attenuators on the HR & OC ultraviewers. This will ease the task of optical cavity alignment by having much better control of the HeNe intensity. Also the prototype of the User Lab laser status panel was completed.  The necessary pig tails and small circuit boards are being fabricated to make installation of the panels streamlined.  Effort continues in User Lab 6 with interlocking the FEL transport system into the LPSS.

    The Charge/Dump Current Monitor PCB was completed and ordered this week.  The parts list for this is also coming together and an order is being setup to ensure all of the parts are in hand.  While the final touches were being completed on the schematic and layout, the firmware for the card was developed.  A basic program has been completed and compiled, this will be tested fully once the pcb is in hand.  Several front panels were ordered this week for the RTD PCBs, beam viewer control cards, 3U processor card, and the charge/dump PCB.  The parts kits for the RTD and Beam Viewer card have been put together so the pcb assembly can take place.  We are using the RTD boards as a test bed for the SIPAD (Solid Solder Deposit) assembly procedure.  This process allows us to place all of the components on the PCB at once and then bake the board in an oven.  This reduces the labor overhead by two thirds.  Since this technique is new to us lots of time was spent to understand the process and the costs.  Twenty boards were shipped out this week to SIPAD for the process.

    Some time was spent with the Magnet Test Group and the DC Power Group about the Sextupole test stand for magnet characterization.  A plan has been worked out to get the power supply and control system install in the magnet test area next week.  All of the necessary components are being gathered and organized so a smooth installation can occur.   

Additionally this week, work has begun this week to produce a better "Video/AMS switcher" screen. The approach to do this requires some re-work of the underlying structure, but the functionality will remain unchanged and the result will be a more intuitive and user friendly screen that will adapt well to growth and change. Time was spent investigating the drive laser pulse control (DLPC) system's behavior in-and-out of vernier mode. We have decided not to plan for any further changes in the software until after we have implemented a list of updates that are scheduled to be implemented with the next reboot. The system will be rebooted over the weekend and we will then re-investigate vernier mode behavior. Additional online system data graphics are under construction this week as well. Current focus is on the FEL Drive Laser. Similar to the FEL operations graph and the plot of the vacuum system, a customized plot of the drive laser data is currently under-way.   The Safety Warden Advisory Team met this week to review the draft for part 1 of the training program that we developed.  The required revisions are being made.  The follow-up for part 1 and review of part 2 is scheduled for next week.

    Spent time this week getting Matt Stokes started on the LPSS controls.  We are currently preparing an application to run on our test IOC for application and screen development.

    The graduate research poster presentation for the Virginia Council of Graduate Schools (VCGS) at the Library of Virginia in Richmond went very well on Tuesday. Although my principal role was to showcase the type of research being conducted at Christopher Newport University (CNU), the poster made a clear point that the FEL at Jefferson Lab "is" the enabling factor for this work in terahertz synchrotron radiation. The invited audience included; US Senators-Representatives-Chiefs of Staff, VA House Delegates and Legislative Assistants, VA Senators-Chiefs of Staff, State Council of Higher Education of Virginia (SCHEV) personnel and others. ()

OTR intensity measurements

Last Friday and this week we have done absolute measurements of the OTR intensity at the injector beam energy. Presently the energy is ~ 9 MeV. The purpose of the measurements is to benchmark our calculations of the OTR intensity and to determine whether there is enough light intensity for the bunch length measurements with the streak camera. First we have done the beam measurements with a properly synchronized CCD camera. Later we removed the camera from the beam line and calibrated it with the help of HeNe laser and a power meter. Figure 1a (left) shows the beam profile measured in the injector via ORT. Figure 1b (right) shows the image obtained during the camera calibration. The power in the calibrating laser spot was about 120 pW. Taking into account the spectral characteristic of the CCD camera, the solid angle in which we collect the OTR, and the vacuum window transmission, the measured and the calculated values are agree. One also has to allow for the error bar for the intensity measurements of about 30 %.

A side product of the measurements is the beam profile measured in the injector, showing the beam profile complexity. We also observe that with the multislit measurements as previously reported.

[pic] [pic]

Fig 1a: Beam profile measured via OTR; Fig1b Calibrating image of 120 pW

WBS 9 (Beam Transport):

Improvements and Upgrades

• New Gun and Injector Test Stand

•  The Power Supply Tank was up righted to the proper orientation in the enclosure and the Alignment Group put it and the gun-mounting stand into the proper position.  Grouting is slated for next week. .

• I spent the week calculating pressure vessel code formulas and preparing the pressure vessel documentation for the SF6 tanks for the New Gun.

•  The JLab Shop is preparing to the weld the flanges on the Gun Chamber. We decided to weld them on before the bake-out because of research by the CEBAF Injector Group that shows that baking a chamber and then welding its flanges on, brought outgassing load to almost pre-bake levels.  We will substitute a bake at lower temperatures for a longer time so that the flange knife-edges will not be softened.  

•  Danny Machie, the designer, is adding modifications and as-built features (from the existing assembly) to the gun’s resistor assembly drawing package (Running Resistor and integral Voltage Conditioning Resistor) for a procurement cycle.

•  McAllister Technical Services says that they are almost finished fabricating the stalk retraction mechanism.  They await the delivery of the welded bellows on the 15th of February to complete the assembly.

SF Sextupole Magnet Testing

•  We continued our plan to test the magnets with the power supplies in Magnet Test with the actual software. Initially we will use a hall probe on the pole tip on magnets off the test stand to show what (1) ramp rate, (2) what dwell times are necessary at the tops and bottoms of the loops and (3) how many full or half cycles are necessary to avoid field quality and set ability problems. 

•  What became clear with our more integrated approach is that the new, in-house developed power supplies probably have the potential to bypass the stepwise current ramps imposed by the EPICS control system. These steps have the potential to cause set ability problems by imposing eddy currents that alter the residual fields in the poles faces.  Developments and tests over the next few weeks will show if these ramps can be made smooth. 

• We agreed on hysteresis cycle current extremes of 95 A (power supplies are 100 A maximum) and test points starting at the 92.4 A required to meet 105% of the full field requirement for the magnets.  This gives headroom to the power supplies as well as maintains the original intent of the system.

•   The problems with power supply communications continue being resolved.

Cooled and less resistive Wiggler Chamber

•  Summary of our work to date: The shop has made the second chamber, leak tight and sufficiently flat to fit in the gap of the wiggler at its narrowest jaw setting without touching.  The attachment methods to its strong back were modified to decouple any longitudinal growth due to temperature rise. Thus, the vital beam viewers (that act as our primary laser optical axis fiducial) should not drift in lateral position.

•   We still need to attach copper cooling water tubes to the narrow, top and bottom edges of the chamber.  That process was proven out using a shorter “test’ chamber. We will not warp the 2nd chamber when we attach the tubes.

• What remains untested is the copper coating process required on the inside of the chamber to reduce its heating to about half of the existing chamber.  We plan to apply that copper layer by sputtering.  The test chamber is now readied to try that process.  We have the copper wire, the test wafers to calibrate the process, the insulated feed throughs and the bellows to accommodate expansion of the copper wire.  Don Bullard is our technician for this effort and he is mostly working on the new gun, so advances are slow.    

Experimental program

Experiment to see a pseudoscalar particle that couples to photons   

• The LIPSS Group is analyzing their data and readying the apparatus for the delivery of the high quality camera in anticipation of a future run.

WBS 11 (Optics):

The performance of the optical transport system (OTS) was studied as a function of power on Monday afternoon and Tuesday.  About 2.4 kW of beam was delivered to User Labs 1 & 2, and to the end-of-line power probe.  We found there are two mirrors at the beginning of the OTS that are absorbing power and causing slow beam drift.  Based on calorimetry and what mirrors need to be moved to correct for long term thermal drifts, the mirrors that are moving are the first and second one.  The Optics Group had already planned to install a shield for the second mirror; discussions are underway on how to shield the first.

We also noted changes in the transmitted mode pattern that were consistent with thermal lensing of the outcoupler.  We will take the measurements made to date and plan how to best compensate for it with the collimator.

 

FEL Mirror Development

  This week we lased on the 1.6 micron set so we could study a) the performance of the new outcoupler, and b) the performance of the optical transport system.  Initial lasing on Friday looked good, better than we found last April when we lased on mirrors from the same coating run.  We expected this, since the THz traps that were installed in late June spread out the THz load on the mirror, and put a larger percentage on the shields.  With more optimization on Monday, we found the absorption level stayed constant, and we continued to have better performance than last April, although the lasing efficiency (in kW/mA) was rolling off.  We'll want to explore this further in the week's ahead.  The calibrated IR camera for thermographic measurements had been installed, but needs to be adjusted before we can collect good data.  This will be done once we are ready to run high average power again (about a week from now).  

 

The coating vendor for the "high Q" outcoupler mirrors has been queried about their delivery.  At the time of this writing we believe we will have the mirrors on hand in several weeks.

  

We prepared paperwork to provide beam time to the "Optics Improvement Collaboration" (CSU & UMN), as part of the proposal  CSU is submitting.

Optic

This week we supported the JTO experiments in the OCR and User Lab 1. The four cavity mirror cameras were adjusted to increase their field of view. For this experiment it was necessary to view not only the cavity mirrors but also their corresponding scatter shields. The OC Optical Cavity Mirror Metrology System (OCMMS) was realigned to both the radius of curvature (ROC) camera and the quadrant detector. The OCMMS incorporates an off-axis HeNe laser that illuminates the cavity mirror providing real-time ROC and position diagnostics. Early in the week the Optical Transport System (OTS) Turning Can Four’s remote mirror actuator failed. All the normal attempts to recover this in-vacuum actuator fail. In order not to delay the operations program we enabled the Turning Can Three’s actuators. On Friday, after the completion of the JTO program, we vented the OTS and resurrected the failed mirror actuator. As of this writing we are pumping down the OTS. Taking advantage of this half day shut down, we are installing the phase noise measurement detector below the injector light box.

 Some progress on the Optical Beam Position Monitor (OBPM) was made this week. Despite some engineered mitigation, the in-vacuum motor is still over heating. We are identifying several modifications that should alleviate this temperature rise. After researching stepper motor stall detectors for the OBPM we had no luck in finding a suitable commercially available unit. We are looking into using a vibration pickup coil for stall detection. This is important given that the OBPM uses two sets of gold coated wires that spin across the FEL beam.

Drive Lasers and Diagnostics

Testing on the advanced drive laser was put off this week. Preparatory activities for the upcoming ODU experiment precluded a Lab 6 laser lockup. A drive laser phase compensation system designed in the past for injector test stand was again reviewed. We are the process of procuring some of the key components.  We continued work on EO sampling. A calculation is being performed to determine the optimal system parameters for bunch characterization. The lasers that are needed for this project were checked and we are trying to resolve some existing issues. We also spent time discussing issues on the QE monitor system with the gun group and have made effort to improve the measurement. The Antares drive laser showed degraded stability during operation, the lamps will be changed today.

Terahertz:

Following the THz imaging tests performed last week, we discussed some of the problems observed with the lenses used for imaging onto the camera array with the manufacturer.  They confirmed our suspicions that there are significant problems with the uniformity of the lens material as well as with the quality of the surface and shape of the lenses due to the manufacturing process.  In light of this, we are in the process now of procuring two other types of lenses that we hope to receive and test in March.  These lenses should have much better material uniformity, surface and shape quality, and still provide similar transmission in the THz region of the spectrum.  It will be necessary to use a filter with one of the lenses, though, to block the mid-IR that is emitted from the target due to heating by the THz beam.  This may result in a slightly lower overall THz transmission onto the camera array for that lens, but we should realize better imaging resolution and contrast.

Also, work resumed on setting the vacuum FTIR back up.  A full alignment of the interferometer to the optical table and vacuum systems has been completed.  Alignment of the reference laser used to measure the mirror displacement should be completed today and then testing of the system repeatability will begin so that it can again be used for bunch length measurements and start being used for spectroscopy tests using either the THz beam or a globar as the source.

Finally, Wade Brock has completed most of the setup for his senior thesis work to make characterize the polarizations of the THz beam.

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